With increasing depletion of fossil feedstocks, starting materials based on renewable resources are needed, e.g. as alternatives to terephthalic acid (a compound used in the production of polyethylene terephthalate, PET). PET is based on ethylene and p-xylene which are usually obtained starting from of oil, natural gas or coal, i.e. from fossil fuels. While bio-based routes to ethylene (e.g. dehydration of bio-ethanol) are operated on commercial scale a straightforward access to bio-terephthalic acid remains difficult. FDCA is the best bio-based alternative to terephthalic acid (for further information see: Lichtenthaler, F. W., “Carbohydrates as Organic Raw Materials” in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, 2010).
HMF is a versatile platform chemical. Alkoxymethylfurfurals, e.g. 2,5-furandicarboxylic acid, 5-hydroxymethylfuroic acid, bishydroxymethylfuran, 2,5-dimethylfuran, and the diether of HMF are furan derivatives with a high potential in fuel and/or polymer applications. Some important non-furanic compounds can also be produced from HMF, namely levulinic acid, adipic acid, 1,6-hexanediol, caprolactam, and caprolactone.
FDCA can be co-polymerized with mono-ethylene glycol to give polyethylene furanoate (PEF), a polyester with properties similar to PET.

FDCA is usually obtained starting from fructose and/or other hexoses via a catalyzed, preferably acid-catalyzed, dehydration to key intermediate 5-(hydroxymethyl)furfural (HMF) followed by oxidation to FDCA. In literature, processes are disclosed where esters of HMF are used as precursors to prepare FDCA (e.g. U.S. Pat. No. 8,242,293 B2).

In the dehydration step by-products are formed, depending on the specific design of the process.
Typical by-products of this process are levulinic acid and formic acid (see scheme below) which are formed upon hydrolysis of HMF.
In processes for preparing a mixture comprising 5-(hydroxymethyl)furfural (HMF) (and one or more by-products) or in processes for preparing FDCA known in the prior art, a mixture comprising 5-(hydroxymethyl)furfural (HMF) is prepared by subjecting a material mixture, comprising one, two or more compounds selected from the group consisting of hexoses (monomeric hexose molecules, e.g. fructose), oligosaccharides comprising hexose units, and polysaccharides comprising hexose units, to reaction conditions so that a mixture comprising HMF, water and by-products (for example, levulinic acid and formic acid) results. Under the reaction conditions oligo- and/or polysaccharides are usually depolymerised, and subsequently the resulting monosaccharides, e.g. monomeric hexose molecules, are converted into HMF. Hexoses, oligosaccharides and polysaccharides are typically selected from the group consisting of fructose, glucose, and cellulose.
During depolymerisation oligo- or polysaccharides are usually converted into monomeric hexose molecules by hydrolytic cleavage of the ether bonds connecting the different hexose units in an oligo- or polysaccharide molecule (e.g. cellulose). The products of a typical depolymerization process (monomeric hexose molecules) are present in their aldehyde form.
Typically, according to routines at least in part previously undisclosed, depolymerization is conducted by using a catalyst, preferably in a one-pot-procedure. Typically a hydrophilic solvent is used (in particular water), e.g. in order to increase the amount of dissolved cellulose thus increasing the yield per process run. It is typically advantageous to conduct the conversion of cellulose into HMF by means of a heterogeneous catalyst in order to facilitate post-synthetic workup. In a typical depolymerization process, an aqueous solution is used as a solvent, sometimes comprising 50 wt.-% of water or more, based on the total weight of the depolymerization mixture used.
Alternatively, if monosaccharides are used as a starting material for preparing a mixture comprising HMF, water, and by-products, e.g. di-HMF (5,5′(oxy-bis(methylene))bis-2-furfural), no depolymerisation step is needed.
Monosaccharides produced or provided are typically subjected to a dehydration process, wherein the monomeric hexose molecule is typically transferred by isomerisation (via e.g. ketone-enone tautomerization) into its ketone form which is subsequently converted into its ring form. After ring closure, the formed ring-closed hexose molecules are typically dehydrated (and optionally further isomerised) resulting in a mixture comprising HMF, by-products (e.g. di-HMF) and water. However, water causes undesirable by-products due to hydrolysis of the formed HMF as described above (for example, humins, levulinic acid and formic acid).
Due to the insolubility of specific monomeric hexose molecules (e.g. fructose) in common organic solvents, a typical dehydration process step in the prior art is performed in an aqueous environment so that an aqueous solution comprising HMF and water is obtained as a (crude) mixture. As mentioned above, the presence of water leads to hydrolysis of HMF into by-products (e.g. levulinic acid and formic acid) and therefore decreases the overall yield of the reaction.
Isolation of HMF from such aqueous mixtures is challenging since HMF often undergoes side-reactions, e.g. hydrolysis (see scheme below).

Hence, the (crude) mixture comprising HMF and water is usually contaminated with by-products to a certain degree and separation of HMF from the by-products is not possible with justifiable effort.
The aforementioned disclosures regarding the depolymerization or dehydration step also apply to (i) a process for preparing a mixture comprising 5-(hydroxymethyl)furfural (HMF) and one or more HMF esters and a corresponding process for preparing furan-2,5-dicarboxylic acid comprising the step of further processing said mixture and (ii) a use of a carboxylic acid ester in a process for preparing 5-(hydroxymethyl)furfural and HMF esters according to the present invention as described in detail hereinbelow or for preparing FDCA according to the present invention as described in detail hereinbelow. In particular, the successive steps of depolymerization and dehydration can be used to prepare a mixture as employed according to the present invention.
Different teachings regarding the isolation or preparation of FDCA or HMF, respectively, have been reported in the patent literature:
WO 2008/054804 A2 relates to “Hydroxymethyl furfural oxidation methods” (title). It is disclosed that a high solubility of FDCA in an acetic acid/water mixture (volume ratio 40:60) is achieved, compared to the solubility in pure water (cf. paragraph [0058]).
WO 2013/033081 A2 discloses a “process for producing both biobased succinic acid and 2,5-furandicarboxylic acid” (title).
US 2008/103318 discloses “hydroxymethyl furfural oxidation methods” (title) comprising the step of “providing a starting material which includes HMF in a solvent comprising water into reactor”. The starting material is brought into contact “with the catalyst comprising Pt on the support material where the contacting is conducted at a reaction temperature of from about 50° C. to about 200° C.”.
U.S. Pat. No. 8,877,950 B2 relates to a “method for the synthesis of 5-hydroxymethylfurfural ethers and their use” (title). HMF derivatives are made “by reacting a fructose and/or glucose-containing starting material with an alcohol in the presence of a catalytic or sub-stoichiometric amount of solid (“heterogeneous”) acid catalyst” (see abstract).
U.S. Pat. No. 8,242,293 B2 relates to a “Method for the synthesis of organic acid esters of 5-hydroxymethylfurfural and their use” (title). The corresponding esters are disclosed to be “the condensation product of formic acid or its anhydride with HMF (formioxymethylfurfural), acetic acid or its anhydride with HMF (5-acetoxymethylfurfural), or of propionic acid or its anhydride with HMF (5-propionoxymethylfurfural)” (see column 1, lines 20-24) or of “(iso)-butyric acid” (see column 2, line 43) or of “(iso)butyric anhydride” (see column 2, line 47). Different catalysts have been employed in a corresponding process (see column 3, lines 1-31).
WO 2009/076627 A2 relates to the “conversion of carbohydrates to hydroxymethylfurfural (HMF) and derivatives” (title). A method is disclosed “for synthesizing HMF by contacting a carbohydrate source with a solid phase catalyst” (see claim 1). Furthermore, a method of preparing HMF esters is disclosed starting from a mixture “comprising a carbohydrate source, a carboxylic acid, with or without an added catalyst to provide a reaction mixture” (see claim 4).
WO 2011/043661 A1 relates to a “Method for the preparation of 2,5-furandicarboxylic acid and for the preparation of the dialkyl ester of 2,5-furandicarboxylic acid” (title). A method is disclosed “for the preparation of 2,5-furan dicarboxylic acid comprising the step of contacting a feed comprising a compound selected from the group consisting of 5-hydroxymethylfurfural (“HMF”), an ester of 5-hydroxymethylfurfural, 5-methylfurfural, 5-(chloromethyl)furfural, 5-methylfuroic acid, 5-(chloromethyl)furoic acid, 2,5-dimethylfuran and a mixture of two or more of these compounds with an oxidant in the presence of an oxidation catalyst at a temperature higher than 140° C.” (see abstract). The oxidation catalyst comprises cobalt, manganese and/or a source of bromine (see claims 3 and 4).
WO 2009/030512 A2 relates to “hydroxymethylfurfural ethers and esters prepared in ionic liquids” (title). A method is disclosed “for the manufacture of an ether or ester of 5-hydroxymethylfurfural by reacting a hexose-containing starting material or HMF with an alcohol or an organic acid dissolved into an ionic liquid, using a metal chloride as catalyst” (see claim 1).
Related art are also WO 2015/075540 A1, WO 2009/155297 A1 and WO 2015/056270 A1.